1. Field of the Invention
The present invention relates to gyroscopes and more particularly to a permanent magnet rotor, two degree of freedom gyroscope which utilizes dual-axis sensors and electronic commutations of a single torquer coil for closing the rotor positioning servo loop to obtain two-axis control.
2. Description of the Prior Art
It is known in the art to use permanent magnet motors for driving inertial gyroscope sensing instruments. U.S. Pat. No. 4,818,922 describes a prior art permanent magnet motor drive for a gyro rotor. Permanent magnet motors are utilized in inertial gyroscope sensing instruments where optimum efficiency, rate stability and low run up time are required. The drive motors include a two phase stator, permanent magnet rotor and suitable drive circuitry to start, run up and maintain synchronous speed. The rotor is supported by a hydrodynamic bearing wherein the spinning rotor supplies the pressurized fluid for the bearing.
In conventional two degree of freedom gyros, two separate control paths are utilized using an angle sensor on each axis and a torque transducer acting about each axis. The conventional means of closing the X-Y loops in a two degree of freedom instrument are described in U.S. Pat. No. 4,862,050. In conventional two degree of freedom gyro loop implementations two separate control paths are utilized using an angle sensor on each axis and a torque transducer acting about each axis. The separate angular sensors are used for sensing tilt about stationary X and Y axes and two separate torquers are used for implementing two stationary servo loops to urge the actual rotor spin axis to the nominal or null spin axis. The output signals from the two sensors serve as inputs to the torquer servo control electronics. The servo control electronics extracts and processes the angular displacement information and provides the drive currents to the two torquers to obtain two axes control.
In conventional angular momentum based two degree of freedom gyroscopes which utilize electronic closed loop control law means it is normal to use two separate torquer coils, one for each rate sensitive axis, to urge the gyro float or rotor to null with respect to the gyro housing as sensed by the gyro's position pick-offs which generate a position signal. A significant disadvantage to having separate X and Y torquers is the costly alignment procedure required to obtain orthogonality between the two torquer coils. If there is a misalignment between the torquers, the gyro's performance is impacted. For instance, due to a misalignment between the two torquer coils, when a rate is applied about a single axis there is a deflection about a single axis, however current flowing in one torquer coil will produce a component of torque along both axes. This results in a deflection about both axes when only a single axis needed to be rebalanced. The deflection about the wrong axis will have to be rebalanced causing erroneous rate information about that axis.
The present invention eliminates the use of two torquers and utilizes the disclosed electronically commutated two-axis gyro control scheme and a single torquer to obtain two axis control of the gyro rotor. Utilizing a single torquer coil alleviates stringent alignment procedures, reduces the cost and improves performance. By utilizing a single torquer coil and obtaining orthogonality electronically, as disclosed herein, an improved gyro can be obtained. The circuit of the present invention is particularly suitable for controlling a miniature two degree of freedom gyro as disclosed in U.S. Pat. No. 5,070,289.